There is known in the art a bottom hole motor for driving a rock-breaking tool, comprising a motor unit coupled kinematically with a spindle unit having a housing. Passing inside said housing is a shaft connected with the rock-breaking tool and mounted in the housing with the aid of radial bearings and a thrust one. The thrust bearing is essentially a set of outer and inner races serving to retain balls located therebetween.
Irrespective of the configuration of the working surfaces of prior art races of the thrust bearing, provision is made for tightening the inner races located on the shaft and the outer races located in the spindle unit housing.
Said prior art bottom hole motor is used advantageously in drilling wells of large and medium diameter, however, a multicomponent nature of the spindle unit and relatively large cross-sectional area taken by the thrust bearing elements render the prior art structure unsuitable for drilling wells of small diameter due to impossibility to ensure its reliable and durable operation. This is due to the fact that the utilization of a conventional spindle unit structure when using the prior art bottom hole motor for drilling wells of small diameter calls for a reduced shaft diameter which, in turn, results in the reduction of permissible power developed by the bottom hole motor.
The choice of required shaft diameter from considerations of strength leads to a considerable reduction in the diameter of bearing balls, this leading to further reduction of the service life of the bottom hole motor because of the reduced carrying capacity of the balls used.